Tags are portable devices which are capable of being attached to items or personnel wearable. They can be used, for example, for remotely identifying the items or receiving information therefrom. In many applications, the tags must be compact and be capable of responding after long periods of inactivity, for example where the tags are incorporated into items placed into storage for periods of several years.
Conventionally, tags can be passive devices which modify and reflect interrogating radiation directed thereto from associated interrogating sources. Because the tags do not provide power gain, their operating range from the sources is often limited to a few meters.
Active tags are known which incorporate onboard power sources such as a miniature electrical cell. Such power sources have a limited operating lifetime, especially if they are required to power their associated tags continuously. Moreover, the sources can make the tags unacceptably bulky for some applications, for example where tags are implemented as film strips for incorporating into spines of library books.
Although it is feasible to power tags from radiation incident thereupon, for example using solar cells incorporated into the tags or by inductively coupling energy from associated interrogating sources to the tags, it is not practicable in some circumstances to do this for safety reasons, for reasons of restricted operating range or for reasons of obscuration in the case of solar cells.
The use of received radio radiation for powering electronic tags is known in the art, for example as disclosed in a published patent application no. GB 2 306 081A. In the application, there is described a passive electrical power supply for providing electrical power to an electronic tag, the supply comprising an antenna for converting received radio frequency radiation into a first electrical signal, and a transformer including wire-wound coils for transforming the first signal into a second signal capable of altering the impedance of a field effect transistor (FET). In operation, the FET provides at its drain electrode a quasi half-wave rectified representation of the second signal which is converted to a unipolar signal by a capacitor connected to the drain electrode, the unipolar signal providing a power supply potential for operating the tag. The supply is operable to convert the received radiation into the unipolar signal such that the transformer operates at the frequency of the received radiation received at the antenna. The transformer can optionally be an autotransformer comprising a single wire-wound coil.
A power supply for a transponder is also disclosed in a published patent application no. GB 2 303 767 A. The supply described provides power to a response circuit of the transponder, the supply generating direct current (d.c.) from received electromagnetic energy. The supply comprises a capacitor charged from a rectifier diode, the diode having a characteristic such that its reverse resistance against a reverse current directed at its n region to its p region is lower than its forward resistance against a reverse current directed from its p region to its n region. The diode is thus connected reversely compared to a conventional diode, its anode being connected to a positive plate of the capacitor. The arrangement allows the transponder to remain functional even when the received electromagnetic energy is relatively weak. The required characteristic for the diode can be implemented by the avalanche or tunnel effect. Moreover, a voltage multiplier may be provided by using a plurality of the diodes with associated capacitors for generating higher supply potentials. The supply does not employ any form of transformer for increasing the potential of signals generated in response to receiving the electromagnetic energy.
Piezo-electric transformers capable of stepping up potentials are also known in the art, for example as described in U.S. Pat. No. 5,828,160 and U.S. Pat. No. 5,389,852. Such transformers are operable to resonate at a frequency typically in a range of several tens of kHz to 300 kHz when stepping up potentials. This range of frequencies is considerably less than that used for electromagnetic radiation conventionally employed to interrogate electronic tags, for example 10 MHz to 30 GHz. Although piezoelectric transformers operating at frequencies above 300 kHz can be fabricated, for example 600 kHz, their cost and difficulty of fabrication renders them unattractive for items such as electronic tags.
Non-contact energy coupling schemes employing piezoelectric devices are known in other technical fields, for example as disclosed in a U.S. Pat. No. 5,749,909 concerning medically implanted devices. In the patent, there is described an energy transmission system for transmitting energy non-invasively from an external unit to an implanted medical device to recharge a battery in the medical device. An alternating magnetic field is generated by the external unit and a piezo-electric device in the implanted medical device vibrates in response to the magnetic flux to generate a voltage. The voltage is rectified and regulated to provide charging current to a rechargeable battery in the medical device. In the arrangement, the piezoelectric device is stimulated by the magnetic flux at a resonant frequency of the device, namely in the order of tens of kHz.
The inventor has appreciated that a principal problem associated with tags operated from radiation incident thereupon is that it is difficult to generate potentials on the tags of sufficient magnitude to operate semiconductor integrated circuits incorporated therein. Such circuits frequently require a supply potential of several volts to function.